With increased Internet traffic of recent years, there is an increasing demand for high-speed packet transmission techniques in mobile communication, and an OFDM (Orthogonal Frequency Division Multiplex) scheme is studied as one of the transmission schemes that realize high-speed packet transmission. The OFDM scheme can reduce degradation of performances due to multipath interference by transmitting data streams in parallel using a plurality of subcarriers and providing CP's (Cyclic Prefixes), and is robust against frequency selective fading by adopting error correcting coding.
It is studied to use resource allocation schemes such as an LRB (Localized Resource Block) and DRB (Distributed Resource Block) when this OFDM is applied in downlink and data for a plurality of mobile stations are frequency-domain-multiplexed on a plurality of subcarriers (for example, see Non-Patent Document 1).
With the LRB scheme, the base station performs the frequency scheduling to which subcarriers are allocated adaptively, based on the received quality of each frequency band in the mobile stations, so that it is possible to maximize the multiuser diversity effect and perform communication efficiently. Frequency scheduling is normally performed per resource block (RB) which puts together several neighboring subcarriers located approximately coherent frequency band adjacent to each other, into a block. Therefore, a frequency diversity effect can be hardly obtained.
By contrast with this, the DRB scheme allocates transmission data for the mobile stations in a distributed manner over the entire band of subcarriers, so that it is possible to obtain a high frequency diversity effect. Further, the DRB scheme allocates transmission data regardless of the received quality of each mobile station and therefore cannot obtain the frequency scheduling effect and the multiuser diversity effect as in the LRB scheme.
On the other hand, recently, synchronous HARQ is under study (for example, see Non-Patent Document 2). Synchronous HARQ is a hybrid-type packet retransmission control method (HARQ) that reports control information (transmission parameters) only upon the first transmission. Synchronous HARQ reports control information only upon the first transmission and therefore can reduce the overhead of control information upon retransmission.
Further, in Non-Patent Document 3, a retransmission control method for switching a modulation scheme per number of retransmissions in synchronous HARQ is under study.    Non-Patent Document 1: “Physical Channel Structure and Procedure for EUTRA Downlink”, 3GPP RAN WG1 #42 meeting (2005.8) R1-050884    Non-Patent Document 2: “Downlink Synchronous Hybrid ARQ Scheme”, 3GPP RAN WG1 LTE Adhoc meeting (2006.01) R1-060103    Non-Patent Document 3: “Redundancy Version and Modulation Order for Synchronous HARQ”, 3GPP RAN WG1 LTE Adhoc meeting (2006.01) R1-060175